This invention relates to a method of preventing polymer scale deposition on polymerization reactor inside surfaces, such as the inside wall of the polymerization reactor, the agitator or impeller shaft and blades, the baffle plates, the temperature measuring device and so forth, in the polymerization of a monomer or monomers having an ethylenic double bond and/or a monomer or monomers having diolefinic double bonds and, at the same time, improving the peelability or removability of a polymer scale deposited on such surface, if any. It further relates to a method of improving the scale inhibitor (scale deposition preventing agent) adherence to the wall surface of glass-lined polymerization reactors.
Emulsion polymerization, suspension polymerization, bulk polymerization, solution polymerization and vapor-phase polymerization techniques are known for the polymerization of one or more monomers having an ethylenic double bond and/or one or more monomers having diolefinic double bonds. In each case, however, polymer scale deposition causes various disadvantages, such as a decrease in heat transfer efficiency, a decrease in product yield, a decrease in product quality due to contamination with scale fragments, an increase in labor required for scale removal and cleaning, and a decrease in time efficiency.
So far a number of attempts have been made to prevent such polymer scale deposition. Thus, for instance, a method known for the suspension polymerization of a halogenated vinyl monomer, such as vinyl chloride, or a monomer mixture consisting of such monomer and a minor amount of a monomer or monomers copolymerizable therewith comprises coating polymerization reactor inside surfaces in advance with a polar compound, such as a quinone compound, an amine compound, a quinone-amine compound, a ketone compound, an aldehyde compound or the like to form a coat film thereon.
However, the effect of such polar compound cannot last long. In particular, when an acrylic or methacrylic ester monomer is a main component of the monomer mixture to be polymerized, the effect of the polar compound as scale inhibitor will disadvantageously decrease markedly, in extreme cases almost to nil, during polymerization.
A presumable reason is that acrylic and methacrylic ester monomers have a very high degree of dissolving power as compared with halogenated vinyl monomers. Thus, monomer mixtures the main component of which is an acrylic or methacrylic ester monomer dissolve partly or wholly those coat films formed by conventional scale inhibitors for the polymerization of halogenated vinyl monomers. As a result, their polymer scale deposition preventing effect is reduced markedly and, in addition, the scale inhibitor dissolved contaminates the product polymer and may cause discoloration of the same. The dissolving power of acrylic and methacrylic ester monomers is still greater than that of aromatic monomers such as styrene and .alpha.-methylstyrene.
Although the dissolving power of acrylic and methacrylic esters decreases in the presence of a halogenated vinyl monomer, which is a poor solvent for quinone-amine compounds, the presence of 60% (by weight; hereinafter the same shall apply) or more of an acrylic or methacrylic ester monomer in the monomer mixture allows almost complete dissolution of the prior art scale inhibitors, hence makes it impossible to achieve a scale deposition preventing effect.
The prior art scale inhibitors thus cannot be used in the polymerization of monomer mixtures containing 60% or more of an acrylic ester monomer and/or a methacrylic ester monomer. However, in most of the commercial cases in which an acrylic ester monomer and/or a methacrylic ester monomer is subjected to polymerization, the monomer mixtures contain 60% or more of such monomer. It is therefore very important from the industrial viewpoint to develop a scale inhibitor which is effective also in such a polymerization system.
The present inventors have already proposed a method of polymerizing acrylic and methacrylic esters which comprises, for the purpose of preventing such polymer scale deposition, coating polymerization reactor inside surfaces, such as the inside wall of said reactor, the agitator, the baffle-plates and so forth, in advance with a solution in an organic solvent of a quinone-amine compound having a molecular weight of at least 3,000 as obtained by subjecting an aromatic diamine and an aromatic quinone to addition reaction in a solvent, or a mixture of solvents, having a solubility parameter of 9.0 to 12.2 (hereinafter such quinone-amine compound is briefly referred to as "quinone-amine compound") and/or a reduced form of said quinone-amine compound to form a coat film (Japanese Laid-open Patent Publication No. 61-7309).
However, this class of quinone-amine compounds is not fully satisfactory, for the scale deposition preventing effect is not satisfactory in certain polymerization systems, for example in polymerization systems for producing polyacrylates or polymethacrylates in which an acrylic or methacrylic ester or an acrylic and/or methacrylic ester mixture is the only polymerizable component in the monomer composition to be submitted to polymerization. Scales which have once deposited are firm and solid and are very difficult to remove.
The present inventors made intensive investigations in an attempt to increase the scale preventing effect without encountering such disadvantages or difficulties and, as a result, found that a hydrophilic coat film formed on reactor inside surfaces by coating these with a coating composition comprising a quinone-amine compound having an average molecular weight of at least 3,000 as obtained by subjecting an aromatic diamine and an aromatic quinone to addition reaction in a solvent, or a solvent mixture, having a solubility parameter of 8.5 to 24.0 and/or a reduced form of said quinone-amine compound and a silica organosol and then removing the organic solvent and the like by drying can produce a remarkable scale deposition inhibiting effect even in those polyacrylate and polymethacrylate-producing polymerization systems for which the above compound, when used alone, has no satisfactory scale preventing effect. It was also found that even if scales have deposited, they have good removability or peelability. Based on these findings, the present invention in its first aspect has been completed.
Meanwhile, in the polymerization for producing acrylonitrile-butadiene-styrene copolymers (ABS), butadiene-styrene copolymers (SBR), acrylic ester-methacrylic ester-butadiene-styrene copolymers (MBS) and the like, glass-lined polymerization reactors are generally used because scales are formed in large quantities and the scales are rich in adhesiveness and therefore difficult to remove.
When such glass-lined polymerization reactors are new, scales can deposit only in small amounts and scale removal is easy. When they are old and show an uneven glass lining surface, however, scales can deposit in large amounts and scale removal becomes difficult.
The prior art scale inhibitors and the quinone-amine compounds previously proposed by the present inventors have insufficient adhesiveness to glass-lined polymerization reactors although their adhesiveness to stainless steel polymerization reactors is sufficient from the practical point of view.
Conceivable as methods of causing such scale inhibitors to adhere to the glass lining surface are the method comprising using an organic-to-inorganic coupling agent, the method comprising providing an undercoat of a resin, such as a fluororesin or a silicone, and the method comprising coating with such a resin admixed with a scale inhibitor. However, each method has its drawback; while the adhesiveness of the scale inhibitor to the glass lining surface can be increased, the scale preventing effect is reduced markedly or, in extreme cases, almost no scale preventing effect can be produced.
The present inventors made intensive investigations in an attempt to increase the adhesiveness of scale inhibitors to the glass lining surface without encountering such disadvantages or difficulties and, as a result, found that a hydrophilic coat film formed on the glass lining surface by coating said surface with a coating composition comprising a quinone-amine compound having an average molecular weight of at least 3,000 as obtained by subjecting an aromatic diamine and an aromatic quinone to addition reaction in a solvent, or a solvent mixture, having a solubility parameter of 8.5 to 24.0 and/or a reduced form of said quinone-amine compound, a silica organosol and an alkyl silicate and then removing the organic solvent and the like by drying can have good adhesiveness to the glass lining surface, produce an improved scale preventing effect and render scales, if any, readily removable. Based on these findings, the present invention in its second aspect has been completed.